There has been known a reluctance motor which comprises a stator having a plurality of magnetic poles, and a rotor rotatably supported in concentricity with the stator and having a plurality of salient poles, and in which the rotor is rotated by a magnetic attractive force produced between the magnetic poles of the stator, which are sequentially excited, and the salient poles of the rotor opposed thereto. Reluctance motors are, however, disadvantageous in that their application is limited. Namely, there are examples that a reluctance motor is practically used as a direct-drive type drive source for a robot arm, and as a small-sized stepping motor, but it is difficult to employ a reluctance motor for an application requiring high-speed rotation, an automobile drive power source having a battery as a power supply, or the like.
More specifically, in reluctance motors, the exciting coils have a large inductance and the magnetic energy stored in the exciting coils is extremely large. Accordingly, substantial time is required for the storage and extinction of the energy, thus delaying the rise and fall of the current and causing a reduced-torque (reduction in torque) and a counter-torque. The reduced-torque and the counter-torque increase with an increase in the rotation speed of the motor. Further, in reluctance motors, the number of times the magnetic energy is stored and discharged per magnetic pole during one rotation of the rotor is large as compared to a three-phase Y-connection type direct current motor. Due to the inductance of the exciting coils, the exciting current is increased at the last stage of the current supply to the exciting coils, and the current supply in this stage does not contribute to the generation of output torque, causing a large Joule loss. As a result, the efficiency of the motor is lowered and the rotational speed is extremely reduced. If, in particular, the number of salient poles and magnetic poles is increased or the gaps between the salient poles and the magnetic poles are reduced, with a view to increasing the output torque of the motor, the rotational speed is significantly decreased. Namely, in such a case, the time required for the rise and fall of the exciting current is further lengthened due to the stored magnetic energy. Also, in the case of increasing the exciting current or using a battery of about 12 to 24 volts as a power supply, the rotational speed of the motor is reduced.
Furthermore, while the torque curve (based on N and S magnetic poles) of a direct current motor having a magnet rotor is symmetrical, that of a reluctance motor is asymmetrical. Namely, in reluctance motors, an extremely large torque is produced when the salient pole approaches a magnetic pole, whereas the torque produced when the salient pole moves away from a magnetic pole is small, and therefore, the output torque of the motor is subject to pulsation. If, following the ordinary operating technique of direct current motors, a reluctance motor capable of producing an output torque proportional to the exciting current even in a state in which the magnetic flux passing the magnetic pole is saturated is used to permit an operation in a magnetic flux-saturated state, the inductance of the exciting coil is varied largely before and after the saturation of the magnetic flux, making it difficult to control the exciting current.
To increase the output torque of a reluctance motor, if the number of magnetic poles and salient poles is increased, the structure of the motor becomes complicated, bidirectional current supply to the exciting coils cannot be effected, and a plurality of systems are required for the exciting coils, making it necessary to use an expensive current supply control circuit. Furthermore, if a converter or inverter, for example, is used for a direct current power source device to rectify the output from an alternating current power supply, the motor is increased in size and in cost. In addition, only those portions of the input alternating current voltage near the peak value thereof are used for the current supply, namely, a pulsed current having a high crest value is supplied, and accordingly, a large electrical noise is produced at the start and interruption of the current supply, and a smoothing capacitor having a large capacitance, accordingly, large-sized and expensive, is required. The pulsed current supply entails a burden on the power feed side and thus is undesirable.
Moreover, in reluctance motors, a large magnetic attractive force which does not contribute to the generation of the output torque is produced between the magnetic poles and the salient poles and causes a vibration.